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STPM33 RMS calculation

Question asked by mszulc on Jan 24, 2017
Latest reply on Jul 25, 2017 by Gary schoombie


I'm working on an energy meter system using STPM33. I designed and assembled PCB. For testing I wanted to use STPM3x Evaluation Software. I can communicate with STPM, I can read and write registers. To see if it works properly I wanted to measure power and energy drawn by 100W resistive load (with phase angle close to 0 or cosp ~= 1) from ACmains (230V RMS, 50Hz). Below is a screen from evaluation software:




Voltage RMS and power (both WB and FUND) calculation appear to be OK (230V and around 100W). However current RMS value is too small, approximately 2 times smaller than it should (430mA measured in with ampere meter in circuit). I'm using 10mOhm shunt resistor as current sensor. To verify that I took C1 RMS Data [16:0] from dsp_reg14 (entire register value is 00D319E7) and using Table 15 (STPM3x current voltage LSB values) from STPM3x datasheet and chapters and from UM2066 User Manual (Getting started with the STPM3x) and calculated IRMS = 0.21707A.

I tried to measure different values of current (from 80mA to 2A) and every time RMS value calculated by STPM33 is around 2 times smaller than value shown by ampere meter. Why is that?

I also don't understand why active power is calculated correctly (100W) while current RMS value is wrong.

Kind regards